Sawn lumber in standard dimensions is the major construction material used in framing homes and many commercial structures. The available old growth forests that once provided most of this lumber have now largely been cut. Most of the lumber produced today is from much smaller trees obtained from second growth forests and, increasingly, from tree plantations. Intensively managed plantation forests stocked with genetically improved trees are now being harvested on cycles that vary from about 25 to 40 years in the pine region of the southeastern and south central United States and about 40 to 60 years in the Douglas-fir region of the Pacific Northwest. Similar short harvesting cycles are also being used in many other parts of the world where managed forests are important to the economy. Plantation thinnings, trees from 15 to 25 years old, are also a source of small saw logs.
Whereas old growth trees were typically between 0.6 m to 1.8 m in diameter at the base (two to six feet), plantation trees are much smaller. Rarely are they more than two feet (0.6 m) at the base and usually they are considerably less than that. One might consider as an example a typical 35 year old North Carolina loblolly pine plantation tree on a good growing site. The site would have been initially planted to about 900 trees per hectare (400 per acre) and thinned to half that number by 15 years. A plot would often have been fertilized one or more times during its growth cycle. The typical 35 year old tree at harvest would be about 40 cm (16 in) diameter at the base and 15 cm (6 in) at a height of 20 m (66 ft). Trees from the Douglas-fir region would normally be allowed to grow somewhat larger before harvest.
American construction lumber, so-called "dimension lumber", is nominally 2 inches (actually 38 mm (11/2 inches)) in thickness and varies in nominal 2 inch (51 mm) width increments from 31/2 inches to 111/4 inches (89 mm to 286 mm), measured at about 12% moisture content. Lengths typically begin at 8 feet (2.43 m) and increase in 2 foot (0.61 m) intervals up to 20 ft (6.10 m). Unfortunately, when using logs from plantation trees it is now more difficult to produce the larger and/or longer sizes and grades in the same quantities as in the past.
The smaller trees of today's forests pose additional challenges for the sawmill. Because of their smaller diameter there is inherently a higher percentage of waste in converting them to rectangular lumber. While this waste is often converted into pulp chips, these are of inherently much lower value than the lumber. Also, the physical geometry of the logs is a cause for additional waste. The logs tend to have considerable taper and often have sweep as well. Sweep is longitudinal curvature along the tree. Defined otherwise, it is the deviation from a straight line of the concave edge when the log is allowed to assume its natural position on a flat surface. While occasional occurrences of extreme sweep will occur, most often it will not exceed about 100 mm in a log about 5 m long (about 4 inches in 16 feet). If logs are squared in a conventional manner prior to lumber manufacture, there is significant additional waste from sweep removal. Sawmilling machinery has recently been developed to saw logs parallel to the sweep curvature. Typically the logs are first oriented with the greatest curvature up or down ("horns up" or "horns down") and parallel faces produced on the sides by saws or chipper heads. They are then turned on one of these flat sides and sawn "around the curve". Surprisingly, the resulting boards, though originally containing the curve of the sweep, will flatten during drying. Average lumber recovery using around the curve sawing may approach 12% greater than by using conventional methods. Exemplary equipment for around the curve sawing is shown in U.S. Pat. Nos. 4,633,924 to Hasenwinkle et al. and 4,653,560 to Wislocker et al. Sawmill equipment for around the curve sawing is commercially available from McGehee Equipment Company, Ukiah, Calif. and other vendors.
Veneers have been "sliced" from prepared cants or flitches for many years. In the past, slicing has been limited to thin products, rarely more than about 3 mm in thickness. Since no sawdust is produced in slicing, conversion from flitch to useable product is high. The typical slicer cuts veneers transversely from flitches; i.e., across the width rather than along the length. Often these veneers are from fine hardwoods and are used for furniture, cabinetry, paneling, or in other applications where appearance is important. In many cases these hardwoods may be extremely rare and expensive. Rosewood or walnut would be examples. Sliced veneers enable a product to appear as if it was made from solid wood but at a small fraction of the cost of a solid wood product. Many decorative treatments are possible with sliced veneers that would not be practical or possible with solid sawn woods; e.g., book matched panels.
Rotary cut veneers peeled in a continuous ribbon from logs are primarily used in the production of plywood. This method is less often used for production of thin decorative veneers. Rotary veneer if used as a surface layer is normally used for products of lower ultimate value than those made with sliced veneers. The undistinguished flat grain is esthetically less pleasing than the appearance of sliced veneers. Due the lathe checks produced when the log is peeled, and other restraints, rotary cut veneers are not available in thicknesses much in excess of about 6 mm (1/4 inch).
In order to increase conversion percentage of sawlogs to lumber, researchers have over the years looked at methods of kerfless cutting; i.e., cutting by some method that does not use saws and produce wasteful sawdust. An early example would be U.S. Pat. No. 3,327,747 to Collins. High energy lasers have also been suggested for kerfless cutting, as in U.S. Pat. No. 4,402,574 to McConnel. Unfortunately, until recently no practical method has been found other than the manufacture of rotary cut or sliced veneer and the available veneer thickness has limited its usefulness in lumber products. An exception might be found in products such as those described in U.S. Pat. No. 3,813,842 to Troutner where plies of rotary veneer cut to the maximum practical thickness of about 6 mm are laid up with the grain direction parallel to produce lumber-like products.
The picture has changed in recent years as slicers capable of cutting slats up to about 20 mm (3/4 inch) in thickness have become commercially available. In contrast to slicers for producing decorative veneers, these generally feed the flitches longitudinally against a fixed knife rather than transversely. The resulting slats have minimal structural damage, such as checks or tears, but may come out cupped or twisted by internal stresses so that they require a subsequent flattening treatment. This may be accomplished by mechanical deformation or by the use of restraint applied during drying. After drying and flattening, the slices may be laid up into panels and the panels subsequently ripped longitudinally to produce lumber-like products in a known manner. Exemplary machines of this type are described in U.S. Pat. Nos. 4,825,917, 5,052,452, 5,318,083, 5,390,716, 5,400,843, and 5,427,163 to Gonner or Goinner et al; 3,783,917 and 5,010,934 to Mochizuki and Mochizuki et al. respectively; and Pat. No. 5,088,533 to Binder. In U.S. Pat. No. 4,977,940, Gonner et al. shows a device for straightening boards or slats produced on slicers of the above type. Gonner describes a composite wood member produced from sliced slats in U.S. Pat. No. 5,069,977 as do Traben and Gonner in U.S. Pat. No. 5,352,317.
It is normal in using the above slicers to use cants or flitches that have been squared; i.e. formed into rectangular parallelepipeds in which each face is at 90.degree. to its adjacent faces. Because of this, considerable wood is lost from the outside of the log in forming a rectangle of the largest possible cross sectional area from the particular cut being formed into a flitch. This waste includes that due to sweep and taper which must be cut out and used for fuel or other lower value products.
The present method is directed to a process for producing composite lumber products using methods that eliminate much of the waste caused by the sweep and taper naturally present in sawlogs.